1. Field of the Invention
The invention concerns an intruder detection system, particularly an improved system for detecting and signaling the presence of a human being within one or more defined areas.
2. Prior Art
Numerous systems have been designed and are presently in use to sense and signal the presence of an intruder within a defined area. Some such systems are active in nature, producing a signal within or to define the secure area, the signal being affected by the presence of an intruder and this affect being employed to actuate an alarm or other signal. Not only do such active systems require a continuous energy input, but their presence also can be deteced by the intruder, permitting them to be avoided or otherwise negated. Other intrusion detection devices are of a passive nature. Most passive systems detect the presence of an intruder in a defined area by sensing and responding to visible or invisible radiation of the intruder. Rossin U.S. Pat. No. 3,839,640, dated Oct. 1, 1974, presents an example of an intruder detection sensor responsive to infrared radiation.
Intrusion detection systems are subjected to a number of nonintruder-related inputs, all of which must be distinguished from an intruder related input if the system is to prove practical in use. Inputs which can cause false alarms included reflected sunlight such as from an airplane or automobile or light being directed onto the detector from automobile headlights or a flashlight out of the detection area. An intermittent electric heater, or flickering light bulb, or fluttering foliage or drape in the detector's field of view, also can actuate present systems, causing a false alarm. But the system must not just avoid false alarms, it must also detect the presence of an intruder, even when the intruder is taking steps to avoid detection, such as wearing white clothing while crossing a white background or while crossing a monitored background only after having adjusted his external temperature to closely approximate that of the background temperature. In short, detecting the presence of an intruder while not producing numerous false alarms presents a significant problem, one which has only been partially solved by previous intruder detection systems.
Many intruder detection systems are battery-powered. Often, such systems employ a circuit to detect battery voltage and to produce an alarm when that voltage drops below a certain level. This, too, results in a false alarm, since the alarm does not signal the presence of an intruder, but rather a drop in battery voltage below a given level.
3. Objectives
An object of the present invention is to provide an improved intruder sensor system uniquely designed and adapted to avoid producing an alarm signal in response to nonintruder related inputs, but to generate an alarm signal only in response to the presence of an intruder within a monitored area. Another object is to signal a drop in battery voltage below a given level, but not to produce a false alarm as a result of such a voltage drop. These and other objects will be apparent to those skilled in this field from the following detailed description of a preferred embodiment of the invention.